A conventional optical sensor is designed to ensure a gain by applying logarithmic compression to two photodiode currents by diodes and then performing differential comparison outputs (see, e.g., JP 2006-294682 A (PTL1) and JP S57-142523 A (PTL2)).
Among other conventional optical sensors are those in which photocurrents of two photodiodes are amplified by Darlington transistor to obtain high gain (see, e.g., JP H11-345995 A (PTL3)).
Photointerrupters to be used for object detection are provided in multiplicity as much as several tens, for example, in copiers in which paper moves through various sites within the copier. For this reason, lower-cost, higher-performance optical sensors are desired.
Under these circumstances, there may be conceived methods in which two photodiodes are used and diode compression and differential comparison are followed to obtain gains as shown in PTL1 and PTL2. However, diodes have a temperature characteristic of −2 mV/° C. so as to be incapable of providing a sufficient voltage as acquisition output signal at as high temperatures as 100° C. or so, problematically. As a result, depending on succeeding-stage circuit construction, there can occur malfunctions or operational fluctuations.